The foiling of an alleged plot by al-Qaeda to detonate a 'dirty bomb' in the US comes at a time of increasing criticism of the CIA and the FBI for perceived intelligence lapses before 11 September.

BBC New Online's world affairs correspondent Paul Reynolds looks how US law enforcement agencies are being forced to change their way of operating in the wake of the attacks on the US.

How much of a success is the "uncovering" of the 'dirty bomb' plot?

On the face of it, this is a tremendous success. The official story is that 31-year-old Abdullah al-Muhajir was trained by Al Qaeda in Pakistan and was arrested when he returned to the US on 8 May.

One theory is that a senior Al-Qaeda operative named Abu Zubaydah, who is being held in Guantanamo Bay, is the source of a tip-off to US intelligence agencies.

The arrest comes at a time of severe criticism of the CIA and the FBI for not sharing and analysing information properly before 11 September. So it should take some of the heat off them. It shows that they are on the case in an effective way.

But, as with all things to do with intelligence, a success today can be overtaken by a failure tomorrow - and vice versa. It is an ongoing war.

Isn't the arrest a bit convenient given the criticism faced by the FBI and CIA?

Some people have, indeed, questioned the timing of the announcement. The FBI and CIA have been criticised in Congress for not "joining up the dots" before 11 September. But, equally, if the intelligence agencies have scored a hit with this arrest, there is no reason to hold it back.

The US public has been deeply concerned about the risk of further attacks and to know that something is being done to stop them is something of a reassurance, although the knowledge that Al Qaeda is still on the march is obviously a continuing fear.

What lessons have been learnt from 11 September?

The FBI lacked a central clearing house for the kind of intelligence it received.

The CIA , which tracked Bin Laden and his network abroad, cannot work within the United States, and was also limited by constitutional protections from passing on everything it might have known.

In the World Trade Center case, we now know that there were several clues, which perhaps needed a central brain to bring together.

The lesson is to try to set up a system to sort the wheat from the chaff. Huge amounts of information come in. How do you know what is true and what is false?

How will the new Homeland Security department help in the fight against terror?

The name of the game now is to centralise and the Homeland Security department will have control over a number of government security and border agencies as well as a coordinating role with the FBI and CIA.

The department will be elevated to cabinet rank

The coordination of intelligence has never been easy in the US, with the CIA designed for intelligence gathering abroad and the FBI for counter intelligence at home. With Al-Qaeda, there is no such distinction as the organisation moves back and forth.

The British have a body called the Joint Intelligence Committee, which brings together all the intelligence agencies and acts as a clearing house.

Such a centralisation has its dangers as well. Before the Falklands War, the JIC did not predict an Argentine move. There is a danger that such a body will stifle independent thought and produce a certain mindset. The Israelis suffered from a such a mindset in 1973, when they didn't think that Egypt would attack.

How will the FBI's new 'super squad' deter future attacks?

To try to achieve better co-ordination, the FBI is organising a 'super squad' of some 1600 agents.

It will hire language and other experts as well as using regular agents. This is to improve the cultural understanding of who might be behind the threats.

It will be in Washington and include a new Office of Intelligence, headed, according to the New York Times, by a former CIA official.

This is a smack in the face for the FBI's New York bureau, which has previously controlled such investigations.

Who is examining what went wrong before 11 September?

The Senate and House Intelligence oversight Committees are carrying out their own investigations.

These committees are hopping mad. Expects fireworks. There is nothing that fires up the Congress like government agencies which it thinks fell down on the job.